U.S. patent application number 12/305484 was filed with the patent office on 2010-05-27 for monitoring and controlling hydrocephalus.
Invention is credited to Andreas Linninger.
Application Number | 20100130884 12/305484 |
Document ID | / |
Family ID | 38895187 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100130884 |
Kind Code |
A1 |
Linninger; Andreas |
May 27, 2010 |
MONITORING AND CONTROLLING HYDROCEPHALUS
Abstract
Systems and methods for monitoring cerebral spinal fluid (CSF)
based on electrical impedance measurements are disclosed. The
systems can include an excitation source of alternating current
(202), at least two sensor electrodes (212,214) adapted for
disposition within CSF in a ventricle of a subject's brain, and an
impedance measuring device (204) electrically connected to the
sensor electrodes (212,214) to measure impedance of CSF. Methods
for controlling hydrocephalus are also disclosed and such methods
can include the steps of disposing an impedance sensor (902) within
CSF in a ventricle of a subject's brain, measuring impedance of the
CSF with the sensor (902), and withdrawing CSF when the impedance
measurement is less than a threshold value.
Inventors: |
Linninger; Andreas; (Oak
Park, IL) |
Correspondence
Address: |
NUTTER MCCLENNEN & FISH LLP
SEAPORT WEST, 155 SEAPORT BOULEVARD
BOSTON
MA
02210-2604
US
|
Family ID: |
38895187 |
Appl. No.: |
12/305484 |
Filed: |
July 2, 2007 |
PCT Filed: |
July 2, 2007 |
PCT NO: |
PCT/US2007/015368 |
371 Date: |
February 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60818095 |
Jun 30, 2006 |
|
|
|
60899243 |
Feb 2, 2007 |
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Current U.S.
Class: |
600/547 ;
600/561 |
Current CPC
Class: |
A61B 5/053 20130101;
A61B 5/031 20130101; A61B 5/0538 20130101 |
Class at
Publication: |
600/547 ;
600/561 |
International
Class: |
A61B 5/053 20060101
A61B005/053; A61B 5/00 20060101 A61B005/00 |
Claims
1. A system for monitoring cerebral spinal fluid (CSF) comprising:
an excitation source of alternating current, at least two sensor
electrodes adapted for disposition within CSF in a ventricle of a
subject's brain, and an impedance measuring device electrically
connected to the sensor electrodes to measure impedance of CSF.
2. The system of claim 1 wherein the system further comprises a
controller adapted to signal for withdrawal of CSF when the
impedance measurement is less than a threshold value.
3. The system of claim 2 wherein the threshold value reflects an
impedance difference between CSF and tissue of the subject's
brain.
4. The system of claim 1 wherein the system further comprises at
least four sensor electrodes adapted for disposition within CSF in
a ventricle of a subject's brain.
5. The system of claim 1 wherein the system further comprises a
pressure measuring device adapted to measure pressure within the
ventricle.
6. The system of claim 5 wherein the system further comprises a
controller adapted to signal for withdrawal of CSF when the
pressure measurement is greater than a threshold value.
7. The system of claim 1 wherein the impedance measuring device is
adapted to measure a voltage drop between the sensor electrodes and
to infer impedance from the voltage drop.
8. The system of claim 1 wherein the sensor electrodes are
separated by a distance in a range of about 10 mm to about 40
mm.
9. The system of claim 1 wherein the excitation source is adapted
to provide a frequency in the range of about 0.01 Hertz to about
135 kHertz.
10. The system of claim 1 wherein the alternating current is about
100 .mu.A.
11. The system of claim 1 wherein the system further comprises at
least two excitatory electrodes adapted for disposition with CSF in
a ventricle of a subject's brain, for receiving alternating current
from the excitation source, and for creating an electrical field
extending around the sensor electrodes.
12. A method for controlling hydrocephalus comprising: disposing an
impedance sensor within cerebral spinal fluid (CSF) in a ventricle
of a subject's brain, measuring impedance of the CSF with the
sensor, and withdrawing CSF when the impedance measurement is less
than a threshold value.
13. The method of claim 12 wherein the step of measuring impedance
further comprises applying a time-varying electrical signal and
measuring a voltage with the sensor.
14. The method of claim 12 wherein the step of measuring impedance
further comprises determining a voltage drop between at least two
electrodes of the sensor.
15. The method of claim 12 further comprising measuring pressure in
the ventricle and withdrawing CSF when the pressure measurement is
above a threshold value.
16. The method of claim 12 further comprising applying an
electrical field extending around the sensor.
17. The method of claim 12 wherein the threshold value reflects an
impedance difference between CSF and tissue of the subject's
brain.
18. A system comprising: an excitation source of alternating
current, at least two sensor electrodes adapted for disposition
within cerebral spinal fluid (CSF) in a ventricle of a subject's
brain, a measuring device electrically connected to the sensor
electrodes and adapted to measure impedance between the sensor
electrodes, a CSF withdrawal mechanism, and a controller adapted to
signal the CSF withdrawal mechanism to withdraw CSF from the
ventricle when the impedance is less than a threshold value.
19. The system of claim 18 wherein the system further comprises a
pressure measuring mechanism adapted to measure a pressure of CSF
within the ventricle.
20. The system of claim 19 wherein the CSF withdrawal mechanism is
adapted to withdraw CSF from the ventricle when the pressure
measurement is greater than a threshold value.
21. The system of claim 18 wherein the system further comprises at
least two excitatory electrodes adapted for receiving the
alternating current and for disposition with CSF in a ventricle of
a subject's brain.
22. The system of claim 18 wherein the CSF withdrawal mechanism
includes a micro-pump.
23. The system of claim 18 wherein the threshold value reflects an
impedance difference between CSF and tissue of the subject's
brain.
24. The system of claim 18 wherein the sensor electrodes are
separated by a distance in a range of about 10 mm to about 40
mm.
25. The system of claim 18 wherein the excitation source is adapted
to provide a frequency in the range of about 0.01 Hz to about 135
kHz.
26. The system of claim 18 wherein the alternating current is about
100 .mu.A.
27. The system of claim 18 wherein the measuring device is adapted
to measure a voltage drop between the sensor electrodes and to
infer impedance from the voltage drop.
Description
BACKGROUND
[0001] The technical field of this invention concerns monitoring
and controlling hydrocephalus.
[0002] Hydrocephalus is a neurological condition caused by the
abnormal accumulation of cerebrospinal fluid (CSF) within
ventricles, or cavities, of the brain. Hydrocephalus, which can
afflict infants, children, and adults, arises when normal drainage
of CSF in the brain becomes blocked in some way. Blockage of the
flow of CSF consequently creates an imbalance between the rate at
which CSF is produced by the ventricular system and the rate at
which CSF is absorbed into the bloodstream. This imbalance
increases pressure on the brain and causes the brain's ventricles
to enlarge. Left untreated, hydrocephalus can result in serious
medical conditions, including compression of the brain, atrophy of
neural tissues, impaired blood flow, coma, and death.
[0003] Hydrocephalus may be treated by surgically inserting a shunt
system to divert the flow of CSF from the ventricle to another area
of the body, such as the peritoneum or another location in the
body, where CSF can be absorbed. Typically, shunt systems remove
excess CSF using a two-part catheter system that includes a
ventricular catheter and a drainage catheter. The ventricular
catheter can have a first end that is inserted into the skull of a
patient and disposed within the ventricle of a patient, and a
second end that is typically coupled to the inlet portion of the
shunt valve. The first end of the ventricular catheter can contain
multiple holes or pores to allow the CSF to enter the shunt system.
At the other end of the shunt system, the drainage catheter has a
first end that is attached to the outlet portion of the shunt valve
and a second end that is configured to allow CSF to exit the shunt
system for reabsorption into the blood stream.
[0004] Generally, the shunt valve, which can have a variety of
configurations, is effective to regulate the flow rate of fluid
through the shunt system. In some shunt valve mechanisms, the fluid
flow rate is proportional to the pressure difference at the valve
mechanism. These shunt valve mechanisms permit fluid flow only
after the fluid pressure has reached a certain threshold level.
Thus, when the fluid pressure is slightly greater than the
threshold pressure level, the fluid flow rate is relatively low,
but as the pressure increases, the fluid flow rate simultaneously
increases. Typically, the shunt valve allows fluid to flow normally
until the intracranial pressure has been reduced to a level that is
less than the threshold pressure of the shunt valve, subject to any
hysteresis of the device.
[0005] Some shunt valves allow external adjustment of the threshold
pressure level at which fluid flow will commence. For example, the
shunt valve can contain a magnetized rotor to control the pressure
threshold of the valve. A physician can then use an external
adjustment mechanism, such as a magnetic programmer, to adjust the
pressure threshold of the shunt valve. However, these magnetized
rotors can be unintentionally adjusted in the presence of a strong
external magnetic field, such as during a magnetic resonance
imaging (MRI) procedure. Unintentional adjustment of the pressure
threshold could lead to either the overdrainage or underdrainage of
CSF, which can result in dangerous conditions, such as subdural
hematoma.
SUMMARY
[0006] Systems and methods for monitoring cerebral spinal fluid
(CSF) based on electrical impedance measurements are disclosed. The
systems can include an excitation source of alternating current, at
least two sensor electrodes adapted for disposition within CSF in a
ventricle of a subject's brain, and an impedance measuring device
electrically connected to the sensor electrodes to measure
impedance of CSF. Methods for controlling hydrocephalus are also
disclosed and such methods can include the steps of disposing an
impedance sensor within CSF in a ventricle of a subject's brain,
measuring impedance of the CSF with the sensor, and withdrawing CSF
when the impedance measurement is less than a threshold value.
[0007] Implementations of the invention can include one or more of
the following features. The system can include a controller adapted
to signal for withdrawal of CSF when the impedance measurement is
less than a threshold value. The threshold value can reflect an
impedance difference between CSF and tissue of the subject's brain.
The system can further include at least four sensor electrodes
adapted for disposition within CSF in a ventricle of a subject's
brain. The system can further include a pressure measuring device
adapted to measure pressure within the ventricle. The system can
further include a controller adapted to signal for withdrawal of
CSF when the pressure measurement is greater than a threshold
value. The impedance measuring device can be adapted to measure a
voltage drop between the sensor electrodes and to infer impedance
from the voltage drop. The sensor electrodes can be separated by a
distance in a range of about 10 mm to about 40 mm. The excitation
source can be adapted to provide a frequency in the range of about
0.01 Hz to about 135 KHz. The alternating current can be about 100
.mu.A. The system can further include at least two excitatory
electrodes adapted for disposition with CSF in a ventricle of a
subject's brain, for receiving alternating current from the
excitation source, and for creating an electrical field extending
around the sensor electrodes.
[0008] Implementations of the invention can include one or more of
the following features. The method can further include measuring
impedance by applying a time-varying electrical signal and
measuring a voltage with the sensor. Measuring impedance can
further include determining a voltage drop between at least two
electrodes of the sensor. The method can further include measuring
pressure in the ventricle and withdrawing CSF when the pressure
measurement is above a threshold value. The method can further
include applying an electrical field extending around the sensor.
The threshold value can reflect an impedance difference between CSF
and tissue of the subject's brain.
[0009] According to one aspect of the invention, a system includes
an excitation source of alternating current, at least two sensor
electrodes adapted for disposition within CSF in a ventricle of a
subject's brain, a measuring device electrically connected to the
sensor electrodes and adapted to measure impedance between the
sensor electrodes, a CSF withdrawal mechanism, and a controller
adapted to signal the CSF withdrawal mechanism to withdraw CSF from
the ventricle when the impedance is less than a threshold
value.
[0010] Implementations of the invention can include one or more of
the following features. The system can further include a pressure
measuring mechanism adapted to measure a pressure of CSF within the
ventricle. The CSF withdrawal mechanism can be adapted to withdraw
CSF from the ventricle when the pressure measurement is greater
than a threshold value. The system can further include at least two
excitatory electrodes adapted for receiving the alternating current
and for disposition with CSF in a ventricle of a subject's brain.
The CSF withdrawal mechanism can include a micro-pump. The
threshold value can reflect an impedance difference between CSF and
tissue of the subject's brain. The sensor electrodes can be
separated by a distance in a range of about 10 mm to about 40 mm.
The excitation source can be adapted to provide a frequency in the
range of about 0.01 Hz to about 135 KHz. The alternating current
can be about 100 .mu.A. The measuring device can be adapted to
measure a voltage drop between the sensor electrodes and to infer
impedance from the voltage drop.
[0011] One or more of the following advantages can be provided by
one or more aspects of the invention. Hydrocephalus can be treated
by monitoring volume of CSF in a ventricle of a subject's brain and
discharging an amount of CSF when the volume exceeds a certain
amount. The volume can be monitored by measuring impedance of CSF
in a brain ventricle and discharging CSF from the ventricle when
the impedance drops below a threshold value, indicative of a
desired ventricular volume typically predetermined by the subject's
physician. Discharging excess CSF based on an impedance measurement
does not or is less likely to depend on gravity, a subject's
physical position, or a subject's exposure to magnetic fields such
as those generated by MRI machines than other hydrocephalus
treatments such as using pressure-based measurements in the brain
to determine CSF discharge from a ventricle. In addition to using
impedance-based measurements to discharge CSF, one or more other
hydrocephalus treatments such as using pressure-based measurements
to discharge CSF can also be used to further improve treatment.
[0012] Other advantages and features will become apparent from the
following description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic diagram of a CSF volume monitoring and
control system according to the invention.
[0014] FIG. 2 is a another schematic diagram of a CSF monitoring
and control system according to the invention further illustrating
the electronic components.
[0015] FIG. 3 is a graph plotting impedance versus volume.
[0016] FIG. 4 is a schematic of a sensor calibration system.
[0017] FIG. 5 is an inductive volume sensor.
[0018] FIG. 6 is a measurement and control system circuit.
[0019] FIG. 7 is a block diagram of a feedback control loop.
[0020] FIG. 8 is an active control screen.
[0021] FIG. 9 is a flowchart showing a process of monitoring and
controlling hydrocephalus.
DETAILED DESCRIPTION
[0022] Referring to FIG. 1, a CSF volume monitoring and control
system 100 includes a volume sensor 102 disposed in CSF in a
ventricle 104 of a brain 106 of a patient 108. The system 100 can
monitor and control volume of CSF in the ventricle 104 by measuring
impedance of the CSF with the sensor 102 and electrically
communicating the impedance measurement to a controller such as a
central processing unit (CPU) 110. The CPU 110 can determine
whether the impedance measurement is less than a threshold
impedance value. The threshold value reflects an impedance
difference (and thus also a conductance difference) between CSF and
tissue of the brain 106. The threshold value also reflects a
threshold level below which volume of CSF in the ventricle 104
exceeds a set point, typically prescribed by a physician. If the
impedance measurement is less than the threshold value, the CPU 110
can signal a micro pump 112 to remove excess CSF from the ventricle
104. The micro pump 112 can remove excess CSF from the ventricle
104 through a shunt tube 114 and into the patient's abdominal
cavity or other suitable location (e.g., a vein).
[0023] The CPU 110 can include an antenna for telemetry and a
battery re-chargeable via radiofrequency (RF) technology and
transmit ventricle measurement data to a telemetry device 116. The
telemetry device 116 can wirelessly receive data from the CPU 110
and record measurements of the patient's actual ventricle size in
real-time. The telemetry device 116 can communicate the data to a
processor 118 (e.g., a personal computer) which can store, analyze,
and/or display the data in real-time.
[0024] In the embodiment shown in FIG. 1, certain elements are
surgically implanted in the patient 108. The sensor 102 is disposed
in CSF in a lateral ventricle 104. The CPU 110 is located in the
subgaleal space between the skull and scalp of the patient 108. The
micro pump 112 is placed in a burr hole formed by drilling,
scraping, or otherwise creating a hole in the skull of the patient
108. The shunt tube 114 at one end is connected to the micro pump
112. In other embodiments one or more of the elements may be
arranged in a different configuration.
[0025] Referring to FIG. 2, another embodiment of a CSF volume
monitoring and control system 200 is shown. An excitation source
such as an alternating current (AC) voltage source 202 can apply a
time-varying electrical signal such as AC current to a sensor 204
disposed in CSF in a brain ventricle 206. When an alternating
current is applied within a fluid-filled cavity such as the
ventricle 206, the impedance of the fluid within the cavity
corresponds to the volume of the fluid. The impedance can be
inferred by measuring a voltage drop across a certain distance of
the fluid-filled space. The relationship between measured volume of
the fluid (V.sub.f) and the fluid's impedance (Z) can be given
as:
V f = ( .alpha. ) [ 1 Z ] ##EQU00001##
[0026] Referring to FIG. 3, the constant .alpha. represents the
slope of the tangent of measured impedance-volume values 300, 302,
304 as shown on an example graph 306 of impedance versus CSF
volume.
[0027] Referring again to FIG. 2, if a CSF-filled cavity such as
the ventricle 206 is surrounded by a tissue with sufficiently
larger impedance (lower conductance) such as a brain tissue 208,
then the measured impedance of the CSF in the ventricle 206 reading
strongly correlates to the volume of CSF in the ventricle 206.
Accordingly, a decrease in the impedance reading by the sensor 204
directly corresponds to an increase in the size of the ventricle
206 and vice versa.
[0028] The sensor 204 can electrically transmit its impedance
readings to a controller 216. The controller 216 can determine if
each of the impedance readings varies from a threshold impedance
value, typically pre-programmed into the controller 216. If the
impedance reading is greater than or equal to the threshold value,
then the inference is that ventricular volume and the volume of CSF
in the ventricle 206 are within acceptable limits. If the impedance
reading is less than the threshold value, then the inference is
that ventricular volume increased and there is excess CSF in the
ventricle 206. The controller 216 can compute a CSF correction and
transmit the CSF correction to a pump 210 in any manner compatible
with the pump 210. The pump 210 may then remove an amount of CSF
from the ventricle 206.
[0029] The pump 210 can include any pump or non-pump mechanism
capable of being operated by the controller 216 and removing CSF
from the ventricle 206, e.g., a micro pump or a shunt. If the pump
210 is a micro pump, it may have a pumping capacity of up to about
1.0 ml/min, preferably in the range of about 0.3 ml/min to 0.5
ml/min, and have a size less than about 4.times.4.times.1.5
mm.sup.3. The pump 210 may include a nano-pump.
[0030] In addition to controlling the amount of CSF in the
ventricle 206 by monitoring impedance and hence CSF volume in the
ventricle 206, hydrocephalus may be monitored and controlled using
one or more additional treatments. For example, intracranial
pressure (ICP) may also be monitored to control CSF levels.
Multiple ICP monitoring and control systems are commonly available,
and any may be used in the CSF volume monitoring and control system
200.
[0031] For example, the controller 216 may have the capability to
control ICP in the ventricle 206. The controller 216 could receive
ICP information from a pressure measuring device capable of
measuring ICP within the ventricle. The pressure measuring device
may signal the pump 210 to remove CSF from the ventricle 206 when
the ICP measurement exceeds a certain threshold level. In another
example, a pressure-controlled shunt could be used either as or in
addition to the pump 210 in the CSF volume monitoring and control
system 200 as a pressure measuring device.
[0032] By monitoring ICP in addition to ventricular volume, a
patient can receive improved treatment. Some patients suffer from
enlarged ventricles even though their ICP is normal (normal
pressure hydrocephalus). Moreover, a weak correlation between the
ICP and enlarged ventricles may be responsible for the overdraining
or underdraining of CSF that often occurs with existing shunts.
Overdrainage occurs due to an excessive removal of CSF and can lead
to the collapse of ventricular spaces, slit ventricle system, and
occlusion of ventricular catheters used to drain CSF. Underdraining
occurs due to insufficient CSF drainage and can lead to larger
ventricles and higher pressures than desired. Further, impedance
measurements of CSF in the ventricle 206 are based indirectly on
electric fields and are, often unlike ICP measurements, independent
of outside forces like gravity, body position, and magnetic fields
such as those produced in MRI procedures. In light of these
shortcomings, it can be advantageous to measure the ventricular
volume directly instead of relying exclusively on pressure-based
correlation.
[0033] The source 202 may provide AC current at about 100 .mu.A.
Voltage from the source 202 can be in the range of about .+-.0.1-5
V with a frequency in the range of about 0.01 Hz to about 135 kHz
to generate an electric field of about 0.0025 V/m in the fluid
space between electrodes 212, 214. In this embodiment, one
frequency is used to sample the sensor 204, but multiple
frequencies may be used and may increase accuracy of impedance
measurements. Higher excitation frequencies from the source 202
typically provide higher voltage values.
[0034] The controller 216 can include a re-chargeable battery with
a capacity of, e.g., about 0.5 Ah operating at 2 V. One charge can
last about one year, and with multiple re-charges possible, the
battery could last the lifetime of the patient.
[0035] The sensor 204 includes two electrodes 212, 214 for
measuring the impedance of the CSF in the ventricle 206. The
electrodes 212, 214 are aligned linearly at a fixed distance. The
fixed distance should allow for the electrodes 212, 214 to be
immersed in CSF and should be large enough for the electrodes 212,
214 to be enveloped by the electrical field resulting from the
current provided by the source 202. The fixed distance between the
electrodes 212, 214 of the sensor 204 may be in the range of about
1 mm to about 40 mm or in smaller ranges of about 10 mm to about 40
mm, about 20 mm to about 25 mm, or about 1 mm to about 23 mm. The
larger the distance between the electrodes 212, 214, typically the
stronger the electrode potentials.
[0036] The sensor 204 may be disposed in CSF of the ventricle 206
at any alignment relative to the ventricle 206. However, a voltage
drop between the electrodes 212, 214 can be more accurately
measured and the resulting data can have smaller error if CSF is
introduced to the ventricle 206 at an axis perpendicular to the
sensor 204 rather than aligned parallel to CSF introduction. The
sensor 204 can be positioned stereotactically in the ventricle
206.
[0037] The sensor 204 may be made of any material capable of
measuring impedance in the ventricle 206. Examples of materials
include copper, gold, stainless steel, titanium, silver, and
platinum-iridium (Pl--Ir). Ideally, material such as Pl--Ir wire is
used because it is MRI-compatible and bio-compatible for long term
use.
[0038] The sensor 204 is ideally MRI compatible. A strong magnetic
attraction between the MR scanner and the sensor 204 can cause the
sensor 204 to reorient itself within the patient, causing injury.
MRI compatibility also implies lack of distortion to MR images as
well as negligible heat effects even in strong magnetic fields.
Additional factors such as size, mass, and implantation site may
also affect MRI compatibility of the sensor 204, usually requiring
careful tests of the sensor 204 in the scanner field.
[0039] A protectant can be used to insulate and waterproof the
interior of the electrodes 212, 214 to avoid fluid seepage and
potential short-circuiting of the electrodes 212, 214. For example,
silicone rubber may be used an a protectant due to its low immune
and inflammatory response.
[0040] The sensor 204 is shown with two electrodes 212, 214 in the
CSF volume monitoring and control system 200, but the sensor 204
may include more electrodes disposed within CSF in the ventricle
206. If the sensor 204 includes more than the two electrodes 212,
214, two (or more) of the additional electrodes may be excitatory
electrodes capable of receiving a time-varying signal from the
source 202 and creating an electrical field extending around the
electrodes 212, 214. If the sensor 204 includes more than the two
electrodes 212, 214, more than one impedance measurement may be
made by the sensor 204 between different ones of the electrodes,
communicated from the sensor 204 to the controller 216, analyzed by
the controller 216, and/or communicated from the controller 216 to
a processing unit outside the patient's body.
[0041] The sensor 204 can make impedance measurements
automatically. For example, the controller 216, e.g., a data
gathering device integrated in or otherwise in electronic
communication with the controller 216, can gather a voltage drop
signal between the electrodes 212, 214.
[0042] Measurements can be taken at the sensor 204 at any sampling
rate, i.e., four readings per second or one reading every five
seconds. The sensor 204 can store the impedance measurements and
transmit the measurements to the controller 216 at any transmission
rate, e.g., at a rate equal to the sampling rate.
[0043] Calibration of the sensor 204 may be necessary to adjust for
specific conditions unique to an individual patient, e.g.,
individual variations in CSF composition, tissue conductance, and
shapes. Calibration concerns the relationship between a voltage
drop between the electrodes 212, 214 and an absolute volume of the
ventricle 206. A calibration approach adjusts for conditions of an
individual patient and involves comparison of actual sensor
readings to an accurate independent measurement.
[0044] Referring to FIG. 4, one example of a calibration approach
includes comparative calibration setup 400 using a beaker 402.
Specific properties of a sensor 404 may be inferred by submerging
the sensor 404 in a complex shaped object 406 filled with CSF or a
CSF-simulating fluid. The voltage drop measured in the complex
ventricular shape 406 can be compared to measurements obtained in a
reference beaker. The comparison can show whether the measurement
in the "blind" experiment in complex geometry equals the voltage
drop in the beaker.
[0045] Another example of a calibration approach includes deducing
electrolytic properties of CSF by electric circuit simulations.
When modeling a CSF-filled cavity as a network of generalized
resistors, capacitors and impedances, the apparent properties of
the fluid can be discovered by tuning the electric elements
representing the fluid until the measured voltage drops for
different volumes agree with the simulation. In a first
approximation, resistance and conductance of the elements can be
equated to the apparent fluid properties.
[0046] Another example of a calibration approach includes using
precise MRI ventricular volume measurements to calibrate a sensor
reading. This approach can use a geometric reconstruction of
patient-specific brain images. Computational tools can be used for
the accurate geometric reconstruction of patient-specific brain
geometries for the simulations of CSF flow dynamics in normal and
hydrocephalic patients. Curvilinear coordinate systems are
introduced to minimize discretization errors in the transport
equations for rendering complex and distorted geometries such as
the human brain.
[0047] Referring again to FIG. 2, the controller 216 may infer
impedance from some or all of the measurements received from the
sensor 204. For example, the controller 216 may store all of the
measurements received from the sensor 204 for transmission to a
processing unit outside the patient's body but may only compute
impedance on a sampling of the measurements.
[0048] The ventricular system of the brain is composed of four
different communicating cavities that are connected to one another
through narrow passage ways. The sensor 204 measures only the
volume expansion of the ventricle 206 in which it is placed, not
the entire volume of the connected cavities. Thus, if the sensor
204 communicates an impedance drop to the controller 216, and the
controller 216 signals the pump 210 to remove CSF from the
ventricle 206, it is because excess CSF exists in that ventricle
206 (excess CSF may or may not exist elsewhere in the brain).
[0049] Referring to FIG. 5, an embodiment of an inductive volume
sensor 500 includes inner electrodes 502, 504 and outer electrodes
506, 508. The inner electrodes 502, 504 are situated to be between
an alternating electric field generated by the outer electrodes
506, 508. Impedance is measured in the form of a voltage drop
between inner electrodes 502, 504. The voltage drop is proportional
to the volume of an electric fluid (e.g., CSF) around the inner
electrodes 502, 504.
[0050] The sensor 500 can be of any shape or size. In this
embodiment, the sensor 500 is a cylindrical rod with straight ends,
although the sensor 500 may have any configuration, e.g., curved
ends, rectangular rod, etc. The inner and outer electrodes 502,
504, 506, 508 can be assembled in various design configurations,
e.g., a linear arrangement as shown on the sensor 500. Each of the
electrodes 502, 504, 506, 508 can be about 1 mm in length. Examples
of sensors that may be adapted for use as the sensor 500 include
deep brain stimulation mechanisms such as those sold by Medtronic
in the Activa Therapy System.
[0051] The outer electrodes 506, 508 may generate the electric
field around the inner electrodes 502, 504 by, for example,
receiving a time-varying signal such as an alternating current from
an excitation source. The voltage at the outer electrodes 506, 508
should be below the water limit window (-0.6V to +0.8 V) to avoid
hydrolysis when the outer electrodes 506, 508 are disposed in
CSF.
[0052] Referring to FIG. 6, a measurement and control system
circuit 600 shows an example of the CSF volume monitoring and
control system 200 in FIG. 2. Although the circuit 600 is described
with reference to elements included in the example CSF volume
monitoring and control system 200, this or a similar system,
including the same, more, or fewer elements, reorganized or not,
may be used to monitor and control ventricular volume in another,
similar system.
[0053] The source 202 generates alternating current used to measure
impedance of CSF by inferring the impedance from a fluid voltage
(V.sub.F). V.sub.F is transmitted to the controller 216 for
comparison with a reference voltage (V.sub.R). In the circuit 600,
V.sub.R is shown as an input to the controller 216, but V.sub.R may
be pre-programmed into the controller 216.
[0054] The controller 216 determines whether V.sub.F is less than
V.sub.R. If V.sub.F is less than V.sub.R, then the controller 216
can signal the pump 210 with a direct current voltage (V.sub.DC).
The value of V.sub.DC can indicate to the pump 210 how much CSF to
remove. For example, the value of V.sub.DC may trigger pumping for
a certain length of time or signal pumping of a certain volume of
CSF measured by the pump 210 as CSF flows through it. In making the
V.sub.F versus V.sub.R determination, the controller 216 infers
impedance from the voltage values and uses the impedance
measurement to determine whether and how to signal the pump
210.
[0055] Referring to FIG. 7, a block diagram of an example feedback
control loop 700 shows a configuration of elements that can be used
in a ventricular volume monitoring and control system. An
excitation source or disturbance 702 electrically communicates a
signal (i.e., alternating current) in the form of a wave 704 to a
volume sensor 706 disposed in CSF of a brain ventricle. In the
presence of an electrical field generated by the wave 704, the
sensor 706 can make an impedance measurement indicating ventricular
volume. The impedance measurement can pass through a low pass
filter 708 to correct for possible error (e.g., detected pulsations
from blood) before being electrically communicated as a filtered
wave 710 to a controller 712. The low pass filter 708 may have a
time constant of about 100,000 s (.tau..sub.f=100,000 seconds) and
receive a signal from the sensor 706 with a frequency of about 1
Hz.
[0056] The controller 712 analyzes the filtered wave 710 by
comparing it with a threshold impedance value indicating an
acceptable boundary limit of CSF volume in the ventricle including
the sensor 706. The controller 712 can use, for example, a
bang-bang control algorithm, a PI controller, or a P controller to
determine a correction from an actual ventricular volume as
indicated by the measured impedance to a desired ventricular volume
as indicated by the threshold value. The threshold value typically
indicates an impedance level below which CSF should be drained from
the ventricle, but the threshold value may indicate an upper limit
impedance level, with the controller's analysis appropriately
adjusted. The correction may reflect the desired ventricular volume
or a volume less than the desired ventricular volume.
[0057] The controller 712 electrically communicates a signal
indicating the correction to a micro-pump 714. Based on correction
data received from the controller 712, the micro-pump 714 can
remove an amount of CSF from the ventricle including the sensor
706. The micro-pump 714 can electrically communicate a signal
through a process transfer function 716. The output of the process
transfer function 716 can be electrically communicated to the
sensor 706.
[0058] Referring to FIG. 8, an example active control screen 800
shows a schematic that may be used in monitoring ventricular
volume. A user may view, manipulate, and analyze data displayed on
the screen 800 such as a sampling rate of a sensor disposed in CSF
of a brain ventricle, measured impedance values, and an impedance
threshold value. Data on the screen 800 can be obtained from the
sensor, e.g., wirelessly downloaded from a controller in
communication with the sensor. The user may also be able to view
and create analyses of the data on the screen 800, e.g., tables and
graphs.
[0059] The screen 800 is not limited to any particular layout or
configuration. For example, manipulation tools such as pulldown
menus, tabs, buttons, selection boxes, and scrollbars can be
implemented using any similar type of manipulation tool. In another
example, graphs can be presented in any graph format (e.g., bar,
line, pie, etc.) and with any orientation (e.g., with horizontal or
vertical bars). Furthermore, two or more screens may be combined
and presented on a single screen and one screen may be divided into
two or more screens. There may also be additional screens.
[0060] Users may manipulate the screen 800 in any way, e.g., using
a mouse, a touch screen, a stylus, keyboard commands, etc. For
example, a user may move his or her mouse pointer over an icon and
click on the icon to access a particular functionality.
[0061] Referring to FIG. 9, a flowchart shows an example of a CSF
volume monitoring and control process 900. The steps described with
reference to FIG. 9 can be implemented in a variety of ways and may
include one or more additional steps or steps within those
described.
[0062] In the process 900, an impedance sensor is disposed 902 in
CSF in a patient's brain ventricle. The sensor includes at least
two electrodes between which impedance of the CSF may be measured.
An electrical field is applied 904 around the sensor so impedance
of the CSF can be measured 906 with the sensor. The electrical
field may be applied 904 via an excitation source supplying the
sensor with a time-varying signal, e.g., alternating current.
[0063] Measuring 906 impedance includes determining 908 a voltage
difference between the at least two electrodes of the sensor.
Impedance can be inferred 910 from the measured voltage difference.
The measured impedance is compared 912 with a threshold impedance
value, the threshold impedance value indicating an impedance level
below which an excess of CSF exists in the ventricle including the
sensor. If the measured impedance is equal to or greater than the
threshold value, then no action is taken to trigger removal of CSF
from the ventricle. If the measured impedance is less than the
threshold value, then a mechanism capable of removing CSF from the
ventricle is signaled 914 to remove CSF from the ventricle. CSF is
then withdrawn 916 from the ventricle.
[0064] The process 900 continually runs from determining 908 the
voltage difference and comparing 912 the measured impedance with
the threshold value and taking further action as necessary.
[0065] Other embodiments are within the scope of the following
claims.
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